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Nobody knows if or when Russian activist and writer Vladmir Kara-Murza will emerge from the coma he slipped into last week following his suspected poisoning with an "unknown substance".

But Kara-Murza's wife Evgenia is certain he was targeted for his role in convincing the US to impose sanctions on Russian officials over the 2009 death of another vocal opponent of Russian President Vladimir Putin - Sergei Magnitsky.

The 35-year-old remains on life support in a Moscow hospital after suddenly falling ill on February 2 as he prepared to fly home to the US for his daughter's birthday. He survived similar suspected poisoning two years ago.

Vladmir Kara-Murza had been travelling around Russia in recent weeks to promote his documentary. Photo / Supplied

Hours before he fell ill, Kara-Murza had paid tribute to Nemtsov on Facebook, posting a photograph of roses and a portrait of his dead friend with the caption: "We're here. We remember".

Like Evgenia Kara-Murza, Russian opposition MP Ilya Ponomarev believes Putin's cronies are responsible for Kara-Murza's current condition, just as they were allegedly behind the murders of Magnitsky and Nemtsov.

"For me, there is no doubt that authorisation for such a thing and for the (attempted) assassination of such a public figure like Vladimir (Kara-Murza) could have only come from higher circles of the current authorities," Ponomarev told the ABC on Wednesday.

"I am not saying it was ordered directly by (Russian President) Vladimir Putin, but it has to be ordered by somebody very close to him."

Boris Berezovsky was a Russian billionaire who fled to Britain after falling out with Putin.

While in exile he threatened to bring down the Russian president by force.

He was found dead inside a locked bathroom at his Berkshire home, with a ligature around his neck, in an apparent suicide.

However, a coronial inquest into his death recorded an open verdict after his cause of death could not be established.

The British police had investigated several previous alleged assassination attempts against him.

Stanislav Markelov and Anastasia Baburova 2009

Human rights lawyer Stanislav Markelov represented journalists (including murdered reporter Anna Anna Politkovskaya) who found themselves in hot water after writing articles critical of Mr Putin. He was shot by a masked gunman near the Kremlin.

Journalist Anastasia Baburova, who was walking with him, was gunned down as she tried to help him.

Sergei Magnitsky 2009

Lawyer Sergei Magnitsky died in police custody after he was allegedly subject to a brutal beating and then denied medical treatment.

Just before his death he had been hired by British-American businessman William Browder to investigate a multi-million tax fraud against the Russian state which Browder's businesses had become unwittingly involved in.

Magnitsky was allegedly arrested after uncovering evidence suggesting that police officials were behind the fraud.

In July 2012 he was posthumously convicted of tax evasion. Browder successfully lobbied the US government to impose sanctions on those linked to Magnitsky's death, sanctions which have been linked to the suspected poisoning of Vladimir Kara-Murza.

Natalia Estemirova was a journalist who sometimes worked with Anna Politkovskaya (who was also assassinated). She specialised in uncovering human rights abuses carried out by the Russian state in Chechnya.

Estemirova was kidnapped outside her home before being shot in the head and dumped in nearby woodland. Nobody has been convicted of her murder.

Anna Politkovskaya was a Russian journalist who was critical of Putin, accusing him of turning his country into a police state in her book Putin's Russia. She was murdered by contract killers who shot her at point blank range in an elevator in her building.

Five men were convicted of her murder, but the judge found that it was a contract killing, with $150,000 paid by "a person unknown.

Alexander Litvinenko is pictured at the Intensive Care Unit of University College Hospital on November 20, 2006 in London. Photo / AP

Alexander Litvinenko was a former KGB agent who died three weeks after drinking a cup of tea at a London hotel that had been laced with deadly polonium-210. A British inquiry found that Litvinenko was poisoned by Russian agents Andrei Lugovoi and Dmitry Kovtun, who were acting on orders that had "probably been approved by President Putin."

Litvinenko was a vocal opponent of Mr Putin, accusing him of blowing up an apartment block and ordering the murder of journalist Anna Politkovskaya.

Paul Klebnikov 2004

An American investigative journalist of Russian descent, Paul Klebnikov was killed outside his office in a drive-by shooting in Moscow. He was editor in chief of the Russian edition of Forbes magazine. He had written about corruption and the personal lives of wealthy Russians, publishing a list of the country's richest people.

Sergei Yushenkov 2003

Sergei Yushenkov was a Russian politician shot dead as he tried to gather evidence proving the Putin government was behind the bombing of a residential apartment block.

He was killed with a single bullet to the chest just hours after his organisation Liberal Russia had been recognised by the Justice Ministry as a political party.